The absence of a number on the cover has led to all sorts of confusion. Most listings just list the books alphabetically by author and describe them as NN (no number) but then put a number beside the book. Then, when those lists are used, that number suddenly becomes that book's number. But the books were sequentially identified with a combination of letters and numbers.
Until recently I was missing the identifier for three books (L, M and N below) but they are now known. So here is the full list plus images of two of these three books.
A King's Enemies Walsh, J.M.
B Death on Sunday Rhode, John
C Death Before Honour Hume, David
D The Stoat Brock, Lynn
E The Bell of Death Gilbert, Anthony
F Death Pays a Dividend Rhode, John
G Wreckers Must Breathe Innes, Hammond
H Make Way for the Mourners Hume, David
I Spades at Midnight Maddock, Stephen
J Jewel Thief Mills, Arthur
K Death Leaves No Card Burton, Miles
L Wanted for Murder Holt, Henry
M Sweet Poison Penny, Rupert
N Mr Babbacombe Dies Burton, Miles
O The Calendar Wallace, Edgar
P Mr. Mortimer Gets the Jitters Gray, Berkeley
Q Dames Don't Care Cheyney, Peter
R Doorway to Danger Maddock, Stephen
S Death at the Bar Marsh, Ngaio
T Overture to Death Marsh, Ngaio
A1 Seven Dead Farjeon, Jefferson
A2 Belt of Suspicion Wakefield, H. Russell
B1 U-Boat in the Hebrides Divine, A.D.
B2 Secret Weapons Walsh, J.M.
B3 The Mystery of the Smiling Doll Holt, Henry
B4 Conquest Takes All Gray, Berkeley
B5 Death of a Peer Marsh, Ngaio
B6 A Maid and a Million Men Dunton, James G.
B7 This Man is Dangerous Cheyney, Peter
B8 He Packed a Gun Sanders, C.W.
C1 Death at Dyke's Corner Lorac, E.C.R.
C2 Five Aces Hume, David
C3 Aunt Sunday Sees it Through Farjeon, Jefferson
C4 The Man From Peace River Reid, Wallace Q.
D1 Fighting Buckaroo Curran, Tex
D2 The Turquoise Trail Tuttle, W.C.
37 Verdict of Twelve Postgate, Raymond
38 Death on a Boat Train Rhode, John
39 Sunshine Sketches of a Little Town Leacock, Stephen
40 The Nutmeg Tree Sharp, Margery
41 Attack Alarm Innes, Hammond
42 My Own Murderer Hull, Richard
43 Ironsides of the Yard Gunn, Victor
44 Thou Shell of Death Blake, Nicholas
45 The Creaking Chair Meynell, Laurence
46 Vultures Ltd. Gray, Berkeley
47 Goose Feathers Digby, George
48 The Flying Years Niven, Frederick
49 The Sky Pilot Conner, Ralph
50 Treasure Island Stevenson, Robert Louis
White Circle L - 1942
White Circle M -1942